LOL! Actually it is a GooDIY and sounds just fine when you say it quickly!
The tennis ball is a good idea, but may be a liner of foam of the correct thickness glued to the inside of the trunk lid or the sub enclosure itself can do the same job with less of a ghetto feel.
The box may stay in place, but if I remen\mber the makeshift back-and-chassis-breakers called "speedbreakers" are anything like in the old days, you will need to secure it a bit better for sure.